Abrasive article and method of manufacturing the same



Patented Apr. 12, 1938 UNITED STATES- ABMSIVE ARTICLE AND METHOD OI KAN-UFACTURING THE SAME m 0. Martin and Frederick A. Upper, Niagara Falls.N. Y., animals, by memo auignmen to The Carbornndum to. Company, NiagaraFalls.

N. Y., a corporation of Delaware No Drawing. Application April 11, ms,Serlal No. 15,826

8 Claims.

This invention relates to improved abrasive articles and a process ofmanufacturing the same. In particular the invention is concerned with amethod of coating abrasive grain with a resin which yields a newintermediate product for use in the manufacture of improved abrasivearticles, both bonded articles such as wheels and stones, and coatedarticles such as sandpaper, flexible disks and belts and the like.

This application is a continuation in part of our copending applicationSerial No. 718,674 filed April 2, 1934.

Viewed from certain aspects, the invention is in the nature of animprovement on a process which has been widely used and which isdisclosed and claimed in a patent to Harry C. Martin No. 1,626,246. Inits preferred embodiment the invention makes use of a product preparedaccording to the method broadly disclosed in the Martin patent, butpreferably using a liquid resin as a plasticizer instead of furfural.

According to the original Martin process, which is used in thepreliminary steps of the process of the present invention, in preparinga mix suitable for use in the manufacture of bonded abrasive articlessuch as bonded abrasive disks and wheels, the abrasive grain is firstwet with a high boiling resin solvent, such as furfural, and themoistened grain is then mixed with a dry, powdered phenolic condensationproduct in the so-called A stage. By suitably proportioning those threeconstituents of the mixture, a product is obtained which, after suitablemixing and manipulation, consists of abrasive grains coated with a layerof the dry powdered resin and in which there is substantially no looseor unattached bond in the mixture.

Although this process was widely adopted by abrasive manufacturers andin fact, as is well known in the trade, forms the basis of the presentresin bonded abrasive industry, there are certain disadvantages in itwhich are overcome by the method of the present invention.

One of the dlfilculties experienced with the Martin process asheretofore practiced has been that the mixes must be used within a fewhours after they are made because they tend to cake up and set into ahard mass which is useless for the purpose intended. Another defect ofthe process results from .the fact that the resin bond is onlylooselyattached to the grain and when the'mixture is pressed, the bondis, to a considerable extent,- i'orced oil the grain and into theinterstices between the abrasive grains thereby tending to reduce theporosity of the finished article. The extent to which articles can becompactedfor the purpose of increasing their abrading life is alsorestricted. This limitation of the density is occasioned by the. factthat gases are evolved when the articles are heated to cure the bondwhich causes the articles to bloat when the density is so high that thegases cannot escape freely.

We have developed a method by which these limitations and faults of themethods of the prior art may be overcome. When the method of ourinvention is carried out, there is no difllculty with mixes becomingsticky or caking into a solid mass and we have been able to makearticles of a density heretofore unattainable without bloating.

We have found that if a mixture such as is made by the Martin process,preferably using a liquid resin as the plastici'zer, is allowed to standin an atmosphere controlled as to temperature and humidity, as will beset forth in more detail later, the resinous coatings around theindividual abrasive grains undergo a transformation whereby theoriginally loose powdered resin blends with the plasticizing. agent toform firm continuous films enclosing the individual grains, withoutcausing the loose mass to set up into a hard cake. These coated grainsmay then be cold molded, preferably after being moistened with a smallquantity of suitable resin solvent to make them slightly more adhesive,and the formed article may be removed from the mold and cured as hasbeen the practice with the so-called "green mixes prepared by the Martinor other similar processes. 7

We have chosen to designate this step of our process as an aging"process during. which the bond ripens" to form the hard adherenthomogeneous films referred to.

While we do not wish to be limited by any explanation we may offer, webelieve that the ripening of the bond involves first of all ahomogenization of the liquid and solid ingredients of the bond with thesimultaneous evaporation of some of the volatile constituents of thebond and possibly a chemical reaction between the bond ingredients. Wehave found that this homogenization involves primarily three factors;namely, the composition of the mix, temperature and time. 4

The composition of the mix involves consideration both of the proportionof liquid to dry ingredients and the chemical composition of the liquidingredients and of thedry resin, the chemical composition of the liquidingredients apparently being the most important consideration.

For example, we have found that mixes plasticized with furfural are muchmore prone to cake into useless masses than are mixes which areplasticized with liquid resin such as the normally liquid partialreaction product of a phenol and formaldehyde.

The difllculty encountered in ripening the bond is to prevent it fromcaking, and the prevention of caking appears to involve four factors;namely, the vapor pressure of the moisture in the air which is incontact with the mix, temperature, time and the depth of the mass of themix during the aging process.

As is well known ordinary air always contains more or less moisture, theproportion varying from day to day and sometimes from hour to hour. Thismoisture exerts a pressure which is spoken of as the partial pressure ofwater in the air or as the aqueous vapor pressure of the air.

It is also known that .certain salts such as calcium chloride forexample, tend to absorb water from the air until the aqueous vaporpressure of the air reaches a certain value which is dependent upon theproperties of that particular salt.

We have found that abrasive mixes containing a resin bond have a similartendency to absorb moisture and that it is this absorption of moisturefrom the air which causes such mixes to first become sticky and thencake into a hard mass. We have also found, however, that this tendencyto absorb moisture can be either eliminated or sufllciently reduced toprevent mixes from caking, provided the water vapor pressure of theatmosphere in contact with such mixes is kept below certain values whichwe call the hygroscopic vapor pressure of the mix.

In other words, for each mix and for any given temperature there is acritical aqueous vapor pressure which, if equalled or exceeded, causesabsorption or possibly adsorption of water from the surroundingatmosphere to an extent suiiicient to cause the mix to first becomegummy and eventually to harden into a solid cake, while if the vaporpressure of water in the air is below this critical value the bond willripen to form the desired films without caking.

This "hygroscopic vapor pressure of the mixincreases slightly withtemperature up to temperatures of about 85-90 F. for mixes containing aliquid phenolic resin plasticizer at which point the effect oftemperature appears to become more pronounced requiring a lowering ofthe aqueous vapor pressure of the air to prevent caking. As previouslystated we have found it to be lower for mixes plasticized with furfuraland the temperature factor appears to be much more pronounced infurfural mixes than it is in liquid resin mixes. For example, we havefound that furfural mixes cake up if the temperature exceeds about 80 F.whereas we have successfully aged liquid resin mixes at temperatures ashigh as 108 F.

The time required to ripen the bond appears to depend on the other threefactors, namely atmospheric vapor pressure, composition and temperature,and to be particularly affected by the temperature employed in the agingprocess. For example, it requires about two days to ripen an ordinaryliquid resin mix at 60 F., whereas if the aging process is carried outaround 85 F., the bond is sufllciently ripened after about 15 hours.This is presumably because the solvent power of the plasticizing agentas well as the rate of evaporation of the volatile constituents of themix normally increase with temperature.

The aging process should be carried out on a mix which is spread out ina layer not too deep, preferably about 1 or 2 inches deep, although itmay be piled up somewhat higher, particularly if provision is made toturn the mix over occasionally so that the weight of the piled up mix isnot always supported by the same grains because the tendency to becometacky and cake appears first at the bottom of the pile of mix.

If the conditions under which the mix is aged are not suitable for thatparticular mix, it will be found that upon standing, the mix will firstbecome sticky and tacky at the bottom, and will then start to hardeninto a solid cake. This tendency appears to progress from the bottom ofthe mix upward and if.a mix is left over night, for example, underordinary conditions of temperature and humidity, the mix will havecompletely hardened into a solid mass.

On the other hand, if a mix is allowed to stand in a properlyconditioned atmosphere, the individual grains will remain entirely freefrom each other and there will be neither stickiness nor any caking. 1

Intermediate between these two extremes there are conditions wherein themix will become sticky and perhaps cake at the bottom, in the moremoderate cases the cake being sufliciently friable so that it can bereadily broken up. However, if the mix is stirred or loosenedoccasionally and particularly if it is turned upside down, the agingprocess can be carried out under conditions which would otherwiseproduce unsatisfactory results in the form of a sticky mass or even ahard cake. Furthermore, we have found that the tendency for a mass tobecome sticky and to cake appears to be more marked during the earlystages of the aging process and consequently the later stages of theaging can be carried out under conditions which would be unsuitable forthe earlier stages. All these factors are to be taken into considerationin what we may term maintaining the aqueous vapor pressure below thehygroscopic vapor pressure of the mix, which is intended to mean agingunder such conditions or performing such operations on the mix that themix does not set up into a hard cake.

The determination of the point at which the bond is sufiiciently ripenedis best made by pressing an article from the mix and curing it and thedegree of ripening required will correspondingly depend on the densityto which the mix is to be pressed. If the bond has not been sufllcientlyripened, the article will bloat during the curing step which leads us tobelieve that the ripening of the bond involves not only theestablishment of conditions such that the mix does not absorb moisture,but also the evaporation of certain volatile constituents which areinevitably present in a mix of the character described.

It might be said, therefore, that the ripening process comprises atreatment of a green mix whereby the liquid and the solid constituentsof the bond homogenize to form firm, adherent films about the grainswhile at the same time the volatile constituents of the mix are reducedby evaporation and the moisture content of the bond is kept down bymaintaining the aqueous vapor pressure of the air below the hygroscopicvapor pressure of the mix.

While it is impractical to attempt to set down the value of thehygroscopic vapor pressure of all mixes because the value varies withdiflerent mixes, we will illustrate the invention with reference totypical mixes in order to give an idea of the order of magnitude of thisfactor and how it varies with temperature.

Example 1 Nine hundred (900) grams of No. 18 grit fused alumina abrasivegrain were moistened with thirty-two (32) grams of a liquidphenol-formaldehyde condensation product resin in the'socalled A stage".After mixing to assure a uniform distribution of the liquid over thesurface of the grain sixty-eight (68) grams of a dry, powdered, heathardenable "A stage" phenolformaldehyde condensation product resin wasadded to the moistened grain and thoroughly mixed to make a product, asdescribed above, in which substantially all the dry resin was taken upby the grain and which had something of a tacky consistency and was whatis commonly described as wet". This product was then screened, spreadout in shallow pans to a depth of about one inch and allowed to standfor 48 hours in a room the atmosphere of which was maintained at 60:5Fahrenheit and 50% :5% relative humidity. At the end of this time it wasscreened again and was ready for use.

In preparing the mixture, it has been found that the apparent wetnesswill vary with the viscosity and solvent power of the liquid constituentof the mix as well as with the temperature and humidity of the mix roomand with the grit size of the abrasive grain. We therefore make minoradjustments of the proportions of plasticizer and dry resin tocompensate for these variables, as is commonly done in the art, theproportions being adjusted to give a mix which is somewhat wet and inwhich there is no unattached dry resin. Adjustments of this kind arecommonly practiced and are well understood in the art.

Example I l A mix was prepared by moistening 900 grams of No. 16 gritfused alumina with 30 cubic centimeters of furfural and mixing themoistened grain with 100 grams of a pulverized A stage"phenol-formaldehyde resin. The mix was spread out in a layer about 1 /2inches deep and placed in a chamber the temperature of which wasmaintained at 74 F. and the aqueous vapor pressure of the air in thechamber was reduced to 9 mm. of mercury by inserting in the chamber adehydrating agent and circulating the air in the chamber with a fan. Themix was removed after 20 hours and was ready for use.

Example III be ripened.

Example IV A mix of the composition of Example III was aged at 108 F.for 10 hours, the aqueous vapor pressure of the air being 14 mm. ofmercury. While there was a tendency for the mix to become gummy at thebottom under these conditions caking was avoided by stirring the mixafter two hours and again after five hours aging. The following generaldirections based on mixes containing 10% bond and 16 grit abrasivegrains will indicate the, magnitude'of the hygroscopic vapor pressure ofmixes and serve as a guide in carrying out the invention:

1. The hygroscopic vapor pressure of a "furfural mix" increases fromabout 9 mm. of mercury at 60 F. to about 10.5 mm. of mercury at 75 F.

2. The permissible upper limit of temperature of furfural mixes withvapor pressures of 9 mm. of mercury is about F.

3. The hygroscopic vapor pressure of a liquid resin mix" in which theliquid resin was a phenol-formaldehyde resin increased from about 11 mm.of mercury at 60 F. to about 16 mm. at about F. and then decreases toabout 15 mm. at about 108 F.

4. While the upper limit of permissible temperature with such liquidresin mix has not been determined because of the difficulty in keepingthe aqueous vapor pressure of the air low enough at the highertemperatures, apparently temperatures somewhat above 108 F. could beused provided the aqueous vapor pressure could be kept low enough as byfirst cooling the air to remove substantially all the moisture and thenheating it.

In making a molded article such as an abrasive wheel, the aged mix,preferably but not necessarily moistened with cresol in the proportionof 1 cc. of cresol per 1000 grams of mix, is put into a mold, pressed atupwards of 3000 pounds per square inch and the formed article is removedfrom the mold and cured by heat in the manner well known in the art. Forexample, the article may be placed in an oven the temperature of whichis initially 70 F., is raised to 375 F. during a period of 24 hours andis maintained at this temperature for 10 hours.

In general, we have found that the pressure required to develop a givencombination of properties such as density and grade will increase as themix is allowed to stand after the bond has ripened. For example, insteadof applying 3000 pounds per square inch to a mix that had been ageduntil the bond had ripened it would be necessary to increase thepressure to perhaps 5000 pounds per square inch if the mix were kept forseveral days more before it was pressed.

Our invention provides a product which is also useful in the manufactureof coated products such as flexible disks.

Our process is adapted to the usual modifications in composition such asthe inclusion of fillers of various kinds. Other types of abrasive grainsuch as silicon carbide, corundum, and emery may be substituted for thefused alumina of the example, other liquid plasticizers or wettingagents may be used instead of liquid resin, and mixtures of varioussolvents have been successfully employed. Similarly, although our methodis particularly well adapted for use with heat hardenable resins such asphenolic condensation products or certain heat hardenable alkyd resins,it may be appliedto permanently fusible resins such as the so-calledNovolaks, or permanently fusible phenolic condensation products, metastyrene, certain thermoplastic alkyd resins and the like, or to mixturesof resins such as, for example, a phenolic liquid resin anda dry alkydresin or powdered shellac. Other obvious changes in composition andmanipulation may be employed within the spirit of our invention asdefined in the appended claims.

We claim:

i. The method of making resin coated abrasive grains which comprisespreparing a mixture of abrasive grains coated with a pulverized resinand a plasticizing agent therefor and aging the mix at a temperaturematerially below the melting point of the resin and in an atmospherehaving an aqueous vapor pressure not substantially in excess of 16millimeters of mercury to ripen the bond and cause it to form firmadherent films about the individual granules.

2. The method of making resin coated abrasive grains for use as anintermediate product in the manufacture of abrasive articles whichcomprises mixing abrasive grains with a powdered fusible resin and aliquid plasticizer so as to coat the grains with the resin andplasticizer and allowing the mix to stand in a relatively thin layer incontact with air having a temperature and an aqueous vapor pressure suchthat the mix will not cake into a hard mass and for a time sufllcient toallow the resin and plasticizer to combine and ripen to formsubstantially dry films on the grains of such a character that the grainwill not cohere into a hard mass at ordinary room temperature underlight pressures.

3. The method of making resin coated abrasive grains for use as anintermediate product in the manufacture of abrasive articles whichcomprises coating abrasive grains with a bond comprising a dry powderedfusible resin and a resin solvent and aging the coated grains at atemperature materially below the melting point of the resin and in anatmosphere of lower aqueous vapor pressure than the hygroscopic vaporpressure of the abrasive mix to ripen the bond and cause it to form firmadherent films about the individual granules.

4. The method of making resin coated abrasive grains for use as anintermediate product in the manufacture of abrasive articles whichcomprises mixing abrasive grains with a liquid plasticlzing agent,adding a dry powdered resin to the moistened grain in proportions tomake a distributable mix which is substantially dry to the touch and inwhich the individual abrasive grains are coated with the resin and theplasticizing agent and aging the mix at a temperature materially 'belowthe melting point ofthe resin and in an atmosphere of lower aqueousvapor pressure than the hygroscopic vapor pressure of the mix to ripenthe bond and cause it to form firm adherent films about the individualabrasive granules.

5. The method of making resin coated abrasive grains for use as anintermediate product in the manufacture of abrasive articles whichcomprises mixing abrasive grains with a liquid resin, adding a drypowdered resin to the moistened grain in proportions to make adistributable mix which is substantially dry to the touch and in whichthe individual abrasive grains are coated with the resin and the liquidresin and aging the mix at a temperature materially below the meltingpoint of the resln and in an atmosphere oi lower aqueous vapor pressurethan the hygroscopic vapor pressure of the mix to ripen the bond andcause it to form firm adherent films about the individual abrasivegranules.

6. The method of making resin bonded abrasive articles which comprisescoating abrasive grains with a bond comprising a dry Powdered fusibleresin and a resin solvent, aging the coated grains at a temperaturematerially below the melting point oi! the resin and in an atmosphere oflower aqueous vapor pressure than the hygroscopic vapor pressure of theabrasive mix to ripen the bond and cause it to form firm adherent filmsabout the individual granules, forming an article from the mixture, andhardening the bond by heating.

7. The method of making resin bonded abrasive articles which comprisescoating abrasive grains with a bond comprising a dry powdered heathardenable resin and a normally liquid resin, aging the coated grains ata temperature materially below the melting polnt of the resin and in anatmosphere of lower aqueous vapor pressure than the hygroscopic vaporpressure of the abrasive mix to ripen the bond and cause it to form firmadherent films about the individual granules, forming an article fromthe mixture, and hardening the bond by heating.

8. The method of making coated particles for use in the manufacture ofabrasive articles which comprises moistening the surfaces of abrasivegrains with a liquid comprising a normally liquid resin, adding a drypowdered heat hardenable resin in a proportion to form a mixture whichis substantially free from unattached resin, spreading the coatedgranules out in layers not substantially greater than one inch in depth,and exposing them to air maintained at a temperature of approximatelyFahrenheit and an absolute humidity not substantially greater than 50grains of moisture per pound of dry air.

HARRY C. MARTIN. FREDERICK A. UPPER.

